Meaning of the word insidiously in English

What does insidiously mean in English? Explore the meaning, pronunciation, and specific usage of this word with Lingoland.

insidiously

US /ɪnˈsɪd.i.əs.li/
UK /ɪnˈsɪd.i.əs.li/

Adverb

in a gradual, subtle way, but with harmful effects

Example:
The disease developed insidiously, showing no obvious symptoms at first.
His influence spread insidiously throughout the organization.